A spinel (MgO-nAl2O3) sintered body has high transmittance for light in the visible light region to the middle infrared region, and good corrosion resistance and chemical resistance, and is superior in heat resistance, pressure resistance and mechanical strength. Further, the spinel sintered body has advantages that its properties are easily achieved stably compared with fieldstone such as sapphire and it is free from a problem of birefringence.
Therefore, the spinel sintered body is used as a light-transmitting material for a window (light-transmitting window material) disposed in electrical equipment or a detector for transmitting light in the visible light region to the middle infrared region. Specifically, the spinel sintered body is widely used as light-transmitting materials such as a high-temperature sight glass of a container for producing a semiconductor, a window for receiving infrared rays of sensors for applications of the visible light region to the middle infrared region, for example, an infrared detecting sensor of a missile or a flame detector, a light-receiving window disposed in electrical equipment for receiving signals from a remote controller, and a window of a snooperscope (Patent Literature 1).    Patent Literature 1: Japanese Patent Laying-Open No. 2006-315878